In conventional arts, a filter used in a trace detector for detecting explosives and drugs only has effects in filtering water, organic substances and absorbable particles in the migrated gas and reacted gas. Filter medium is filled in the whole inner space of the filter. Due to the sucking action from the gas source, the migrated gas in the flow path of the detector passes through the filter before entering into the migrated pipe. Because of the on-way pressure lost and partially pressure lost of the gas flow caused by the filter, as well as the uncertainty of the arrangement of the particle filter medium and the effects of the inner structure of the filter, the flow current and the pressure of the passing gas is fluctuated. Thus, the detecting performance of the detector is affected and the detecting precision is decreased.